First detection of carbon dioxide in an exoplanet

Finding bodes well in searching for life signatures beyond the solar system

Moving one step closer to finding the fingerprints of life in a habitable planet beyond the solar system, astronomers have for the first time detected carbon dioxide in the atmosphere of a planet that orbits a star other than the sun.

The extrasolar planet and its star lie about 63 light-years from Earth. A gaseous body slightly bigger than Jupiter, the orb circles its parent star at a proximity that renders it far too hot to support life. But the finding bodes well for ultimately detecting carbon dioxide and other potential markers of life in planets that do lie far enough from their parent stars to be habitable, says Mark Swain of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

In the atmospheres of more temperate planets, carbon dioxide — along with water, methane and oxygen — can be generated by biological processes, Swain says. “In that context, the carbon dioxide measurement constitutes a dress rehearsal …for our long-term goal of trying to detect signs of life or signs of habitability on terrestrial-mass planets or superEarths in the habitable zone” around a star, which is where water could exist as a liquid.

Using the Hubble Space Telescope’s Near Infrared Camera and Multi-Object Spectrometer, Swain and his colleagues recorded infrared spectra from the planet, which periodically transits, or crosses in front of, its parent star as seen from Earth. When the hot planet lies side by side with its parent star, HD 189733, astronomers can detect the spectra of infrared radiation, or heat, from both star and planet. When the planet dives behind the star, only the infrared radiation from the star reaches Earth. Subtracting the two measurements gives the amount of infrared radiation given off by the planet alone.

The spectra of the planet’s radiation, recorded by the Hubble instrument, reveal chemical constituents of the planet’s atmosphere. The team found evidence of water vapor, which was previously detected in the planet’s atmosphere, as well as carbon monoxide and the never-before-seen carbon dioxide, the researchers report in an upcoming Astrophysical Journal Letters. Team member Gautam Vasisht of JPL also presented the findings on November 19 in Paris at the Molecules in the Atmospheres of Extrasolar Planets meeting.

According to models that assume conditions in the atmospheres of Jupiter-like planets are similar to those of slightly more massive objects, such as brown dwarfs, carbon dioxide isn’t expected to be present in detectable amounts in such planets.

One explanation for the carbon dioxide, says Swain, is that because the planet lies so close to HD 189733, completing an orbit in just 2.2 days, it receives an unusually high dose of ultraviolet light from the star. The intense ultraviolet radiation could have altered the chemical composition of the planet’s atmosphere, breaking down compounds and creating new ones. If other explanations can be ruled out, “this would be the first real evidence that [ultraviolet starlight] can make a substantial contribution to the atmosphere of these extrasolar planets,” Swain says. In the solar system, for example, ultraviolet light from the sun is believed to have triggered complex chemical reactions in Earth’s early atmosphere.

The fact that the carbon dioxide compounds were identified in relatively low-resolution spectra, comments theorist Alan Boss of the Carnegie Institution for Science in Washington, D.C., “represents the proof-of-concept for what we would hope to look for when characterizing the atmospheres of extrasolar Earths with a Terrestrial Planet Finder-type space telescope.”

Study coauthor Drake Deming of NASA’s Goddard Space Flight Center in Greenbelt, Md., says the carbon dioxide detection in this hot Jupiter-like planet “presages a similar detection in the atmosphere of a rocky superEarth planet by the James Webb Space Telescope at [an infrared] wavelength of 4.3 micrometers.” Set for launch in 2013, the James Webb is the proposed successor to the Hubble Space Telescope.

The new finding “means that three of the Big Four biomarkers for habitable/inhabited worlds have now been seen: water, methane and now carbon dioxide,” Boss says. “The only one that has not yet been detected is oxygen/ozone.”

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